Undertale AU Oneshots

By -_Quickstorm-_

12K 152 54

Firstly, this is something I wrote back in 2017 or so, so don't expect too much out of it, I just thought I'd... More

Some author's notes before we start.
1. Underswap! Sans X Mute! Reader
2. Underfell! Sans X Reader
3. Inktale! Sans X Slightly Corrupted! Reader
4. Errortale! Sans X Reader
5. Babybones! Skelebros X Winged! Monster! Reader
6. Underswap! Papyrus X Neko! Reader
8. Echotale! Sans X Reader
9. Underswap! Sans X Shapeshifter! Reader
10. Undetale! Undyne X Reader
11. Underfell! Papyrus X Reader
12. Birdtale! Sans X Reader
13. The Thought! Sans X Reader [Final Oneshot, I'm sorry.]

7. X-tale! Sans X Gem! Reader

655 7 2
By -_Quickstorm-_

[Just a quick author's note here... I originally wasn't planning to do this oneshot until much later, but I did some reasearch on this AU and it's characters, and I think I can do this. Now, if you guys don't know, I love Steven Universe, so much so that I made more than a few Gem OCs.. *Smiles nervously* Anyway, I just rewatched the episode 'Bismuth', I don't remember what number it is, but it gave me an idea for a oneshot. And, it also made me kind of sad, again. If you don't know what SU is, you don't have to read this, I won't make you. I haven't done a Gem reader until now in this, so.. Anyway, for the storyline of this, X-Tale Sans, or better known as Cross, is the only monster left in his universe, and all the places in the Underground are still the same, because everyone else died, *They were killed* before they had a chance to change anything. No, they weren't killed by Cross, and I can't tell you, either, because I'd be spoiling things, *I actually have no idea what they were killed by..*.. But, for the storyline of this, that's how it's going to be. If you don't like how I'm doing things here, once again, you can skip this one if you want. Anyway, for this, Cross spends his time going to other universes to steal parts of them to rebuild his universe, since when the creature that killed everyone else came, it destroyed a lot. And if you don't know what a Gem is, they're physical forms are made of light, and they don't age, or have to eat, or sleep, and they have a gem somewhere on their body, and the only way to kill them is to shatter it. Oh, and [G/n] stands for Gem name, while [G/c] stands for gem color. Okay, you got it now? Alright. And, this is going to be the only dark one here, but I needed to write a dark one, so sorry... I won't make anything else this dark, at least not in this book..]- Quickstorm


You blinked as you heard quiet footsteps, trying your best to make out anything in the darkness. You lifted your head and pulled yourself up slightly, but flopped back down almost immediately. "Cross?" You asked in a a whisper, and instantly, there was a crash that made you wince, hoping he was okay. "Holy crap, don't do that, [G/n]!" Cross almost shouted at you once he'd picked himself back up, his pupils glaring at you in the darkness. You cringed under his intense glare, giving him an apologetic look. "I'm sorry..." You apologized, quickly avoiding his gaze, since it was becoming too much for you to look at. Cross sighed, and you could tell he was slightly annoyed, but he didn't yell at you, instead he just breathed very deeply to calm himself. The skeleton had found you in his broken universe one day, and he shot bone attacks into you before you could even explain yourself. But, he stopped himself from killing you as he finally saw that you weren't all human, you were also a Gem. He'd just taken care of you from that point on, you never really knew the reason. However, unlike most Gems, you didn't have a weapon. You had healing powers, and you didn't know why this was, but it had stopped Cross from dying once. Cross spent most of his time going into other universes, trying to steal parts of them to rebulid his own. He'd told you that something killed every other monster, and he was the only one left, because he'd managed to kill it, but it had taken his brother, and his friends.


Cross's blood was also strange. When he bled, it wasn't red, but purple. It had suprised you slightly, but you accepted it after some time. You had also seen his Soul, only once. Half of it was an upside-down, white monster Soul, while the other half was part of a red Soul. There was also a human spirit that was trapped within Cross's Soul, and their name was Chara. Though, they rarely showed up when you were around, since you were kind of scared of them. They had been one of Cross's only friends before you came, but you didn't want to know the reason why they were, uh.. fused with Cross's Soul. All you knew was that Cross didn't force them to be a part of him. Cross wore a white jacket with strange attachments and fur on the hood of it, he also wore a white scarf, and his black shorts and slippers had 'X' markings on them, that was why you called him Cross. His real name was Sans. At first when you had called him by the nickname, he had absolutely ticked off at you, but, after some time, he allowed it. The skeleton had been very mean at first as well, always snapping at you every time you passed him, but that was only because no one had ever loved or cared about him before, not since he lost everyone.


Cross's eyes were different colors, as well, which you found strange, but cool. He had one white pupil, and one red. He also had a red, lightning-shaped scar under his red eye, and you'd never questioned it, though to you, the scar made him look cool. However, As for his weapons, Cross could summon a giant knife, and other knife-like weapons. He did have some magic, though, mostly teleportation and bone attacks as well. You had been so lost in thought that you hadn't realized Cross had said something, and you tilted your head slightly in confusion. "What was that?" You asked, and Cross sighed, meeting your gaze. "I said 'It's okay,I'm pretty sure you didn't mean to scare me.'" The skeleton repeated himself, and you mentally scolded yourself for getting so lost in thought. You nodded silently, and Cross turned away, making you notice the weapon he held. His knife. "Cross, where are you going?" You asked, thinking that maybe you shouldn't have asked that just yet. "I have to go take another universe." He said, gripping his weapon tighter as it gleamed with the little light in the room. "Can I come?" You asked, giving him a hopeful look, and Cross moved his gaze to you, his pupils darkening. "No. The universe I'm going to is much too dangerous for you, even if you are a Gem." Cross started to turn away again, but you reached out and gently grabbed him by the jacket sleeve, stopping him again.


"Cross, I really don't want to lose you. I-" Cross silenced you with a dark glare, carefully pulling himself out of your grip. "Fine! You can come with. But if you die, I'm going to be mad at you." You went over to stand next to him, and Cross's red pupil glowed as he started to teleport. "Trust me, Cross, you won't regret this." Cross made no reply to your words as the two of you teleported. You found yourself in Snowdin, and it wasn't as destroyed as the one you lived in. Most of the trees were still there. Cross pulled you close to him, and you stated to question it, but he quickly silenced you. "Hush, I don't want him finding us." The skeleton whispered, his words almost inaudible, and you went silent, not asking who it was. Cross dragged his knife through the snow as you followed him, leaving a long, red mark behind the both of you. It wasn't until a skeleton appeared that you froze, and Cross's eye glowed, as he tensed. "Fell." Cross snapped, and the skeleton moved his red pupils towards the other monster. "Cross. How many times do I need to tell you, this is my universe. You want to DIE?" Fell threatened, and Cross just stood there, his expression unfazed. You kind of regretted not listening to him now, because Underfell was one of the most dangerous universes, and you knew Fell Sans was not to be underestimated. If only you had let Cross finish...


Cross pulled his knife out of the ground, raising it. "I'm pretty sure you'll die first." Fell just glared, and suddenly, bone attacks appeared next to him, causing Cross to charge him, where he was almost immediately impaled. "Cross!" You shouted, your eyes wide as you watched him. The skeleton threw his knife before he fell into a kneel, managing to stab Fell with it. Fell, however, didn't even flinch. He just ripped it out, and threw more attacks at Cross. You could tell that your friend was going to be finished if you didn't do something, and you didn't hesitate to put yourself in front of him, taking the hit. Cross's eyes widened in shock as you winced, an attack through your chest, but not the gem."No, [G/n]!" You just smiled at him, though it was slightly pained. "I love you, Cross..." You managed, before you collapsed with a dull thud, only worrying Cross more. Fell glared at both of you, before he disappeared in a flash of red magic. Cross immediately knelt next to you, gently holding you close to him. The skeleton blinked back purple tears that were forming in the corners of his eyesockets, lowering his gaze. "Please don't leave me, I need you... I don't want to be left alone again.. Please..." He whispered, and though it took everything you had, you reached up and put a hand on his shoulder.


"Cross, it's okay. It was just my physical form, I'm going to be just fine." Even you were part human, more of you was Gem, so you would be able to reform. Cross froze, purple tears running down his face, as he paused. "What..?" He asked, and you smiled at him gently. "I'm going to be okay. Cross, when a Gem's form is badly damaged, they release their physical forms, and retreat to their gems to heal. Though, it may take a while..." You had never explained this to him, but you should have before now. That way he wouldn't have been so freaked out and worried. "Wait, so you're going to reform? But, that means you'll still be gone for who knows how long..." More tears slowly ran down the skeleton's face, and you gently wiped them. "No, Cross, please don't cry, I'm not leaving you! I'll be back, I promise. Just make sure you keep my gem with you. The most it takes is a couple weeks, it'll be okay. I'll see you again. Though it may look like I die when I explode into dust, but I don't. " Cross blinked away his tears, giving you a soft smile. "For however long it takes... I'll wait for you. I promise. I won't give up on you for a second." You were starting to lose your form, but you still managed "Thank you, Cross. I'll be waiting for you, too. Goodbye for now.." It was then that you exploded into dust, and Cross caught your gem before it hit the snow. He stood up, and pressed it close to his chest before he teleported, murmuring a very quiet "Goodbye, kiddo. I'll see you soon."


[Uh... Sorry about that one... It was a lot longer than I intended, too. I was planning to make her gem crack, but, I got a tear in my eye while writing that version, and decided it was too dark and sad. So, I erased it and wrote this. Plus, I would hate it if Cross was just left alone for the rest of his life. He would probably either die in a AU while trying to take a part of it, or he would slowly go insane, having lost the one he loved, and everything would just cave in on him, even if Chara did try to help him. So... I don't want that. I like this a lot better. Anyway, for the next oneshot I'm either going to do Echotale Sans or Birdtale Sans, I'm not sure yet. I also have some ideas for The Thought, but not enough to write a oneshot yet. I'm thinking more along the Echotale one, because I've sort of gone through the story, and I want to write it out. So, prepare yourselves. I'm sorry if I didn't stick to Cross's personality as well, this is what happens when you try to write something from a AU you just did reasearch on. But, I do like Cross, though he's not my favorite of the AU Sanses. I would have to say either Blueberry or Fell for that. And, the reason that the details are sometimes lacking, is because I think of what'll happen and I don't completely go through it like I do with fanfics. I really should start thinking these through a bit more..]- Quickstorm

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